Explaining the Color Rendering Index (CRI) and LEDs

The Color Rendering Index, or CRI, is a quantitative attempt at measuring the ability of a light source to render known colors of objects and materials. The process involves measuring the light from a test source and seeing how the color of reflected light appears compared to eight known color samples. These samples are “known” colors in that they have an expected color when exposed to a reference light source, and the difference between the known color and the actual color is used to calculate the CRI of that source. The type of reference light source, the source of light that produces the “known” color value, will also be different depending on the type of light source that is being tested. For test sources that have a correlated color temperature no greater than 5000 degrees Kelvin, a black body radiator is used as the reference source. For test sources that have a higher CCT, a spectrum of daylight will be used as the reference source. Regardless of the type of light source being tested, it is important to choose the proper reference source, so that accurate CRI information can be retrieved from the sample colors. Over the years, the CRI scale has been an accurate measure of the effectiveness of indoor and outdoor lighting in reproducing expected colors across the spectrum. Recent breakthroughs in lighting technology, however, have produced Light Emitting Diode (LED) clusters, which produce light in a different way than traditional phosphor coated elements. These LEDs tend to score low on the CRI scale, and yet often reproduce color in an appealing way.

LED lighting systems make use of a “cluster” of light emitters, rather than using a single, broad-spectrum emitter. Because of this, the light that comes from an LED cluster doesn’t perform the same way against sample colors in CRI tests. However, LED lighting systems are typically more appealing in their ability to reproduce color, have less harsh ambient light, and are more efficient than phosphor-coated lights. Because of this, there is an active movement in the lighting industry to adjust the CRI when it comes to LED lights. With LEDs becoming the standard for both indoor and outdoor lighting systems, some engineers have proposed that an entirely different scale be used to measure LED lighting, such as the Scotopic-Photopic (S/P) ratio or the Color Quality Scale (CQS).

Regardless of the type of lighting being used, it’s important to make quantitative measurements of lighting both before installation and afterwards. The reason for taking measurements at the site of installation is to ensure that ambient light and environmental effects don’t distort the quality and color reproduction of lighting systems. This can be done through the deployment of light sensing technology such as spectrophotometers and chroma meters, each of which have their own benefits in any light measurement scenario. Because there are many variables that can affect light quality and color reproduction, it’s often necessary to take multiple readings on-site, before a final lighting design can be settled on and deployed.